Gas heating apparatus



Dec. 10, 1968 w. s. WATSON GAS HEATING APPARATUS Filed May 26, 1966FiG.i

United States Patent ABSTRACT OF THE DISCLOSURE An improvement in anapparatus for the production of gaseous plasmas by induction heating isdisclosed. A baffie having peripherally positioned, angularly disposedapertures, is positioned between the gas-feeding means and the inductionheating zone to provide streams of gas flowing in a helical path.

The present invention relates to improved apparatus and process for theproduction of hot gas plasmas by means of induction heating.

The production of hot gaseous plasmas, i.e. gases heated to very hightemperatures (usually by electrical energy) and which contain asubstantial proportion of active species such as thermally dissociatedmolecules and/or ions produced by the removal of electron(s) from themolecules, by means of induction heating is known and t has beendescribed, for example, in our co-pending applications 256,386 and506,109.

In such processes the gas which is to be heated to form the plasma ispassed through a refractory (and generally electrically-non-conducting)gas-confining tube around which is a coil of electrically-conductingmaterial.

When the process is in operation the gas which is to be heated is passedthrough the gas-confining tube and an oscillatory current of suitablepower and frequency is passed through the coil surrounding the tube. Anelectric discharge is induced in the gas (after initiation) by thismeans and a hot luminous gaseous plasma ball is formed within thegas-confining tube beneath the coil around which the oscillatory currentis passed. Where a continuous flow of gas is passed through theconfining tube gas heated to a very high temperature is discharged fromthe outlet from the tube.

Difliculty may be experienced in stabilising the plasma so that itsexistence is maintained, for example, under varying conditions of gasflow and/ or composition of the gas (e.g. when a mixture of gasses ispresent in the plasma).

It is an object of the present invention to provide an apparatus andprocess whereby improved stability is obtained.

Accordingly, the present invention is an apparatus for the production ofgaseous plasmas by induction heating comprising a gas-confining tubesurrounded by an electrically conducting element, the ends of which areadapted for connection to a source of oscillatory current and containingwithin the upper end of the gas-confining tube a bafile plate, theperipheral edge of which is provided with a number of apertures whichare so formed as to divert the flow of gas over the baffle into streamsof gas following a substantially helical path within the gas-confiningtube.

The gas-confining tube is normally made from a refractory andsubstantially electrically non-conducting material, for example silica,at least as far as that portion which encloses the plasma is concerned.It is, however, possible to form the tube from other material, forexample an electrically conducting material such as a metal, providingsteps are taken to prevent its acting as a susceptor. One

3,415,968 Patented Dec. 10, 1968 method of preventing this is by theinsertion of one or more strips of insulating material longitudinally inthe wall of the tube, and/or by limiting the thickness of the wall ofthe tube. Metal gas-confining tubes of this type are described, forexample, in our co-pending application 506,109.

The electrically conducting element may conveniently consist of a coppertube formed into a coil around the exterior of the gas-confining tube.Generally, a space is left between the exterior of the tube and theelement and each turn of the coil is spaced apart from the adjacent turn(s).

The number of turns in the coil may vary as desired but is preferably atleast 3, and particularly at least 5. If desired, the upper and/or lowerturn of the coil (in the direction of the gas flow) may be so formed asto conduct electricity in the reversed direction to the adjacent coil inorder to provide a magnetic field which assists in confining the plasmato the volume within the coil.

During operation of the apparatus a cooling fluid, for example water,may be passed in heat exchange relationship with the electricallyconducting element to prevent its overheating.

The ends of the electrically conducting element are adapted forconnection to a suitable source of oscillatory current. A sourceproviding a power of at least 3 kilowatts and preferably at least 8kilowatts at a frequency from about 10 kilocycles/sec. to aboutmegacycles/ sec. and particularly frequencies in the range 25kilocycles/sec. to 10 megacycles/ sec. has been found very suitable foruse in the process of the present invention.

The baflie plate in the upper part of the gas-confining tube (in thedirection of gas flow through the tube) is preferably a substantiallycircular plate across the tube (i.e. the diameter of which is at rightangles to the longitudinal axis of the tube) and generally of slightlysmaller diameter than the internal diameter of the tube containing it.

It is preferred to make the baflie from material having good heatconducting properties, for example copper, in order that the heatradiated from the plasma can be dissipated.

The periphery of the bafl'le is provided with a number of apertures,preferably spaced equidistantly. The number of such apertures willdepend upon the circumference of the baflle plate since they arepreferably spaced at intervals of about /2" and there are generally atleast 4 and preferably at least 8 such apertures.

The apertures are so shaped that they divert a stream of gas flowingover the baffle (and over the edges thereof) into streams which follow asubstantially helical path within the gas-confining tube. One method ofachieving this is to make the apertures in the form of slots and to bendthe corresponding edge of each of the slots at an angle to the plane ofthe baffle. Generally, where this method is used, the edge of the slotmay be bent downwardly (i.e. in the direction of gas flow) at an anglebetween about and 60, particularly between 40 and to the plane of thebafile. A baffle of this type is described, for example, in theaccompanying drawings.

In addition to the apertures around the periphery of the bafile it mayalso be desirable to provide an aperture in the centre of the baflle,through which can be introduced fluids and/ or finely-divided solids, asdesired, into the plasma.

One convenient method of suspending the baffle in the gas-confining tubeis to fix it to a rod or tube which is, in turn, fixed to a cap orsimilar structure at the top of the gas-confining tube. Where the baffleis supported on a tube (as opposed to a rod) this may pass through thebaffle and its upper end may pass through the cap on the gas-confiningtube, thus providing a conduit leading to the aperture through thebaffle for the introduction of fluids and/or finely-divided solids intothe plasma.

It has been found during operation of the present invention that a backpressure is created upon the battle by the expansion of the gas whenthis is rapidly heated on contact with the plasma. The effect of thismay be reduced, when necessary, by providing one or more additionalapertures through the bafile, for example consisting of holes drilledthrough the baffie between the centre and periphery. These holes arepreferably placed equidistantly on a line concentric with the centre ofthe baflle.

The gas, for example an inert gas, oxygen or a metal halide, which flowsover the baflle and through the apertures around the periphery may beintroduced into the gas-confining tube by any convenient method. Onemethod which has been found to be very suitable particularly when thegas-confining tube is provided with a cap at its upper end, is tointroduce the gas through a pipe or pipes set in the cap. Two or morepipes are preferred which are so positioned as to direct the flow of gason either side of an imaginary plane passing through the longitudinalaxis of the gas-confining tube, for example as shown in the accompanyingdrawings.

It is believed that by the use of a bafile plate according to thepresent invention the flow of gas is so directed i.e. as a number ofstreams following a substantially helical path within the gas-confiningtube that greater stabilisation of the plasma is obtained than has beenpossible hitherto and that greater variations can be tolerated in theflow rates and/or variation in gas composition without extinguishing theplasma.

In the drawings accompanying this specification FIG- URE 1 is adiagrammatic longitudinal section of an apparatus according to thepresent invention and FIGURE 2 is a plan view of the bafile plate.

Referring to FIGURE 1 the gas which is to be heated is introduced intothe gas-confining tube 1 through inlets 2 and 3. The upper part of thegas-confining tube is enclosed by a copper cap 4, the lower end 5 ofWhich is internally threaded to engage with the externally threadedcentre portion 6 of the brass cap 7. Between these threaded portions ofcopper cap and brass cap are placed a brass insert 8 and a siliconeO-ring 9 to form a fluid tight seal with the gas-confining tube.

The outer rim of the brass cap is threaded to engage with internalthreads at the upper end of a Perspex cylinder 10. The brass cap isprovided with an outlet 11 for coolant and an inlet for the coolant 12is also provided in the wall of the Perspex cylinder.

Around the exterior of the gas-confining tube is wound copper tubing 13which is coated with polyvinyl chloride. The ends of the copper tubingpass through the walls of the Perspex cylinder and are provided withmeans for (a) Connection to a source of oscillatory current of theappropriate frequency, and,

(b) Passing a fluid coolant such as water through the tube.

The lower end of the Perspex cylinder is similarly provided with a brasscap 14 through which the gas-confining tube projects. A fluid tight sealis provided between the brass cap and the lower end of the gas-confiningtube.

In the upper end of the gas-confining tube is fixed bafile support tube15 to which bafile 16 is attached.

The periphery of baffle 16 is provided with apertures 17 spacedequidistantly and formed by cutting radial slots in the battle andthereafter bending downwardly the corresponding sides of the slots to anangle of 45 to the plane of the baflle.

Referring to FIGURE 2, this shows the bafile 16 and apertures 17. Thereare also shown holes 18 equidistantly spaced on a line concentric withthe centre of the baflle and approximately halfway between the centreand the periphery of the baflle. These holes serve to reduce backpressure upon the baflle when the apparatus is in use.

Example I A device similar to that described above was set up in whichthe gas-confining tube consisted of a silica tube 1 ID. and 1 OD. and15" in length. The bafile plate was made of copper and was 1 indiameter. Eight apertures were formed around the edges by cutting radialslots 4;" deep and by bending downwardly the corresponding edges to anangle of The bafile plate was also provided with 3 holes /33" diameter.

The baffle plate was fixed from the lower end of the bafile platesupporting tube which was 7 OD. and this was introduced into thegas-confining tube in such a manner that the baffle plate was heldhorizontally in the gas-confining tube A from its upper end.

The copper tubing forming the coils was ID. and 6 coils were provided,each coil being about 7 apart.

Cooling water was passed through the interior of the Perspex cylinderthrough the copper tube. Argon was introduced through the inlet (whichwere at an angle of to the top of the device) at a rate of 40 l.p.m. andthrough the interior of the baffle support tube at a rate of 10 l.p.m.

An oscillatory current having a frequency of 5.3 megacycles/sec. and apower of 8 kilowatts was supplied to the ends of the copper tube and aplasma discharge was initiated in the gas-confining tube beneath thecoils. The flow of argon was then gradually replaced by oxygen l.p.m.through the inlets and 24 l.p.m. through the bafile support tube) andthe power was introduced to 36 kilowatts.

The device operated smoothly during the change-over and for a prolongedperiod thereafter with markedly greater stability than had beenexperienced under similar conditions in the absence of a battle plate.

Example 2 The process as described in Example 1 was repeated using anoscillatory current having a frequency of 5.3 megacycles/sec. and apower input of 36 kilowatts.

The stability of the device in operation was shown by varying the rateof gas flow through the angled inlets between 20 and 200 litres/min. andthat through the baffle support tube between 5 and 40 litres/min. whilemaintaining the plasma in the gas confining tube.

What is claimed is:

1. In an apparatus for the production of gaseous plasmas comprising agas-confining tube, an electrically conducting element in surroundingrelation to said gasconfining tube adapted for the induction heating ofgases within said gas-confining tube when said electrically conductingelement is connected to a source of oscillatory current, and meansassociated with said gas-confining tube for feeding gases into saidgas-confining tube and into the induction heating zone formed withinsaid gas-confining tube by said electrically conducting element, theimprovement which comprises bafile means positioned within saidgas-confining tube between said gas-feeding means and said inductionheating zone, said baffie means provided with peripherally positioned,angularly disposed apertures adapted to divert the flow of gas from saidgas-feeding means into streams of gas of substantially helical flow pathinto said induction heating zone within said gas-confining tube.

2. An apparatus as claimed in claim 1 wherein the baffle means is acircular plate positioned with the diameter substantially at rightangles to the longitudinal axis of the gas-confining tube.

3. An apparatus as claimed in claim 1 wherein said bafile meanscomprises a baflie plate having at least 8 apertures in the peripheraledge thereof.

4. An apparatus as claimed in claim 1 wherein the baflle apertures areformed from peripheral slots by bending the corresponding edges of theslots to form an angle of between 30 and 60 with the plane of thebaffle.

5. An apparatus as claimed in claim 4 wherein the angle between the edgeof the slot and the plane of the baffle is between 40 and 50.

6. An apparatus as claimed in claim 1 wherein said bafiie means isadditionally provided with apertures between the center and theperiphery thereof.

7. An apparatus as claimed in claim 1 wherein said baflie means isadditionally provided with a central aperture.

8. An apparatus for the production of plasma heating comprising incombination, a gas-confining tube; an electrically conducting elementcomprising copper tube coiled in surrounding relation to saidgas-confining tube with at least three coils surrounding a portion ofsaid gas-confining tube, the ends of said copper tube connected to acurrent source; feeding means associated with one end of saidgas-confining tube adapted for feeding gas into said gas-confining tubeand into the portion of said gasconfining tube surrounded by said coils;and bafiie means positioned between said gas-feeding means and theportion of said gas-confining tube within said coils, said bafile meansprovided with at least 8 apertures formed from peripherally positionedslots by bending corresponding edges of the slots to form an anglebetween 30 and 60 with the plane of the bafile.

9. An apparatus as claimed in claim 8 wherein said UNITED STATES PATENTS2,686,621 8/1954 Horn et al. 21910.65X 3,108,169 10/1963 Keller 21910.79X 3,114,626 12/1963 Dombrowski et al. 219-1065 X 3,277,265 10/1966Reboux 219-10.65 X

RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

US. Cl. X.R. 21910.43

